Glossary

A hormone produced in the stomach that stimulates appetite. The body’s production of ghrelin may be influenced by a person�s amount of sleep. See also leptin.

Growth hormone

A hormone secreted by the pituitary gland that promotes growth of the body and influences the metabolism of proteins, carbohydrates, and lipids. The secretion of growth hormone peaks shortly after sleep onset.

Hertz (Hz)

The number of cycles per second; the unit of measurement applied to a rhythmic event, such as brainwaves.

Hippocampus

A part of the brain that plays a central role in many functions, including processing of memories.

Histamine

A substance released by cells that causes symptoms of an immediate allergic reaction. Histamine released in certain areas of the brain promotes arousal. Conversely, anti-histamine medications can promote sleep.

Homeostasis

Maintenance of the body’s internal environment, for instance, through regulation of blood pressure, temperature, water, and nutrient content. This stability in the long term is usually achieved by a control mechanism called “negative feedback,” where short-term changes in a regulated physiological variable are counteracted by reflexes that bring the physiological variable back to a stable set-point level.

Homeostatic sleep drive

The drive to sleep that accumulates during prolonged wakefulness, and lessens during sleep. Sleep homeostasis is one of the primary modulators of sleep in humans.

Hypnagogic

Occurring in the time period surrounding sleep onset.

Hypnagogic hallucinations

Vivid images, sounds, or tactile feelings that occur when drifting off to sleep. The hallucinations can be frightening and are sometimes accompanied by sleep paralysis.

Hypnogram

A graph that summarizes the pattern of sleep stages across a night, for instance, as recorded in the sleep laboratory.

Hypnopompic hallucinations

Vivid images, sounds, or tactile feelings that occur when waking from sleep. The hallucinations can be frightening and are sometimes accompanied by sleep paralysis.

Hypnotic

In sleep medicine, hypnotic refers to an agent that promotes sleep.

Hypocretins

Neurotransmitters that normally promote stable wakefulness and help regulate REM sleep. In people who have narcolepsy with cataplexy, there is a loss of neurons that produce hypocretins. Hypocretins are also known as orexins.

Hypothalamus

A deep brain region just in front of the brainstem that regulates arousal, sleep, hunger, body temperature, and other fundamental behaviors. The neurons that produce hypocretins lie in the hypothalamus.

Insomnia

Trouble falling asleep, staying asleep, or waking up too early.

Insulin

A hormone produced by the pancreas that directs the passage of glucose, amino acids, and fatty acids into cells and promotes their storage.

Insulin resistance

A condition in which the body does not respond normally to insulin. Insulin resistance can lead to type 2 diabetes and heart disease. Current research is exploring potential links between inadequate sleep and elevated risk of insulin resistance and diabetes.

Jet lag

A condition caused by air travel through changing time zones and the resulting desynchrony of the body clock and external time. Jet lag is marked by fatigue, insomnia, nausea, and irritability.